IM for first tri

what is your swim like, especially in open water if you have had any experience?  You can fudge the bike and the run to a degree but if you arent confident in the water, that alone is worth getting a few other tris under your belt so you dont freak out.  I had a great view of the swim at IM Wisconsin last year and saw a few freakouts and wondered what kind of prep those folks had done.  Dunno, maybe they did do their prep and it was just a bad day. 

There are a few people who do either a HIM or IM as their very first triathlon.  Yes, you CAN prepare for one in a year depending on your background, goals, lifestyle, etc. 

No reason you have to do a sprint before doing an IM, they're two entirely different races related only by the fact they have swimming, cycling, and running.  If you're much more inclined to run a marathon than a 5K, it's OK to skip the sprint.  (I would agree with Jzzat about some open water swim experience and that may be the biggest reason to start with something short. ) 

The real question for an Ironman is can you spend enough time training (and recovering) to be ready for the race.  Endurance, pacing, nutrition, and hydration become much more important in the long races.   

The swim is far and away the weakest event for me. I have been spending a lot of time on the saddle this summer aiming at a century in two weeks. The order of my strengths would be bike run swim.
How do you train for an IM or HIM. Is it really a case of training each sport indivudually for the set distance or something else. I envision it being training for a marathon, training for the 112bike and training for the swim and then o nthe day lumping them all in together. This might be a bit naieve.

Find a training plan that makes sense given your goals and time available to train and follow the plan as much as you can. 

Long rides will top out at 85--120 miles.
Long runs will top out at about 3 hours.
Long swims will top out at 4000-5000 yards. 

These are your times to see what works for nutrition, hydration, pacing, etc.  Some people will do a brick run (mine were usually about 3 miles) after a long ride.  Some will do a brick with a shorter ride and longer run (maybe 65/8).  I've even seen plans that call for an "epic day" where you swim for 90 minutes, ride 6-7 hours, run an hour, do housework/yardwork/play with the kids/anything on your feet, then another hour running to finish about the time of your expected race finish. 

Pay attention to your body and take a rest day if you need to rest. 

The nice thing about an IM swim is IF you expect to finish the swim in 1:45-2:00, you can wait 5 or 10 minutes before you start swimming and have relatively clear water. 

As for which one to choose as your first, that's a crazy broad question, which is probably why no one has answered it yet. It totally depends on location, willingness to travel, if you are able to register off-site, time of year, your strengths/weaknesses compared to the course, etc. And since everyone has different goals and abilities, everyone's answers are going to be different.

I'd recommend doing a few things to choose your first. Read some race reports on this site from past Ironman events. See what people liked or didn't about the race and venue. Also, check out the actual web sites for specific information about each race such as lodging, time of year, and course maps.

It is really an individual decision, based on a ton of individual factors.